Reflective mirrors are used in many applications such as telescopes, binoculars, cameras, microscopes, optical instruments and other applications where the concentration, diffusion, magnification or directional change of a light beam is required. Such mirror structures typically comprise a core or substrate made of glass, glass-ceramic, silicon carbide, or a graphite composite, with a mirror or reflective surface formed on or bonded to the core.
It is old and well known in the art to fabricate mirror structures so that they are light in weight. There are many advantages that follow from making mirrors that are as light in weight as possible. For example, light weight mirrors have less thermal mass and therefore do not distort as much and come to thermal equilibrium faster when subjected to thermal transitions. They also are less subject to gravity induced sag or distortion in the shape of the reflective surface, and therefore can produce superior images. Because of their lower mass, light weight mirror assemblies require only minimal support systems, resulting in a reduction in total system mass, size and overall cost. For devices such as binoculars and cameras, a lighter weight and smaller size enhances the handling and transportability of the device. For other devices, such as telescopes and other applications requiring systems for changing the directions of orientation of the optics, a reduction in weight of the mirror permits the use of actuators which have relatively low output forces and which may be more accurately controlled.
In view of the advantages that accompany optical mirror structures that are lighter in weight, various methods and techniques have been developed to reduce the overall weight of the mirrors. Principally these methods and techniques involve reducing the weight of the substrate.
One method of reducing the weight of the substrate is to reduce the amount of material used in the substrate. For example, U.S. Pat. No. 5,076,700 to DeCaprio is directed to a lightweight mirror having a core that is lightweighted by drilling a series of pockets in the core. After the core is machined, a faceplate having a reflective surface thereon is bonded to the remaining ribs and outer wall of the core.
In U.S. Pat. No. 5,227,921 to Bleier et al., there is disclosed an optical assembly having a core that is comprised of two members, each of which has a plurality of ribs. When the ribs of one member are bonded to the ribs of the other member in a crossing, abutting manner, the result is a core having multiple channels therethrough. Other prior art patents that disclose lightweight mirrors or mirror blanks employing channels or honeycombs include U.S. Pat. No. 3,713,728 to Austin et al., U.S. Pat. No. 4,842,398 to Ducassou, and U.S. Pat. No. 5,604,642 to Deminet et al.
Another technique for reducing the weight of the substrate is to utilize light weight materials to form the substrate. For example, U.S. Pat. No. 4,035,065 to Fletcher et al. discloses a substrate formed from a lightweight cellular glass material. The substrate is directly bonded to a reflective member, and the substrate and reflective member have approximately the same coefficient of thermal expansion.
In U.S. Pat. No. 5,208,704 to Zito, there is disclosed a fibrous substrate made from silica and alumina fibers. The fibers contain voids which are sealed by a clay-containing sealant. A glassy layer and then a reflective layer are applied to the substrate to form an ultralight mirror.
Although the prior art techniques are useful for preparing light weight mirrors or mirror blanks, they nevertheless often involve specialized materials or difficult or time consuming machining operations that increase the cost of fabricating such lightweight mirrors. It would therefore be desirable to have a lightweight mirror blank that can be manufactured from relatively low cost common materials at volume efficiencies, yet deliver high quality optics.